Robots that move themselves across prepared tracks, rough terrain, or even submerged surfaces are useful for myriad tasks such as mine sweeping, hull inspection, excavation, reconnaissance, warehouse automation, et cetera. Moving robots have been made in widely varying forms from wheeled vehicles to anthropomorphic walkers having legs with knees, ankles and feet.
Legged robots, in general, do not scale easily to large sizes. The reason is the strong influence of dynamics in their operation. Inertia forces (and moments of inertia) scale as the fourth power of linear dimension. However, the force available from an actuator, such as a muscle or hydraulic cylinder, that provides force in proportion to its cross-section increases as only the square of linear dimension.
A frame walker is a type of legged robot that has two sections connected by a mechanism. Usually each section has three or four legs. A frame walker moves by using the mechanism to lift one section off the ground while resting on the other section. After the lifted section has been moved to a new location, the mechanism lowers it and lifts the other section. The mechanism alternates lifting, moving, and lowering the sections to make the robot take steps. At any given time, the robot is supported by whichever section is not lifted.
Frame walkers are simpler in their scaling behavior than general legged machines. Frame walkers do not use continuous relative rotational motion and they are operated in a statically stable manner with approximately constant velocity. Conventional frame walkers do use relative rotational motion to change direction; that is done slowly to keep inertial moments within acceptable bounds.
In a conventional frame walker two sections are connected by a serial mechanism, meaning one in which actuators for different degrees of freedom are connected serially. (An example of a serial mechanism is a person's arm: wrist motion is connected to shoulder motion serially through the elbow joint.) Serial mechanisms usually offer good range of motion which enables a frame walker predicated on a serial mechanism to take long strides.
Significant room for improvement exists in the field of frame walkers. For example, it would be useful to have a frame walker that could take a step in any direction or that could tip and tilt its lifted section. A design that was not subject to the scaling constraints that affect frame walkers based on serial mechanisms would permit larger size walking robots.